The distribution of a lumpy bulk material over an expansive surface is a problem which is known to specialists in the field of plant construction and process technology. Particularly in the case reactors used in chemical/physical process technology, considerable efforts are being made to achieve a bulk-material distribution which is optimized to a particular process. Incorrect loading of a reactor of this type can lead to a fall in the quality of the product, to high losses caused by the extraction of dust and to a reduction in productivity of the plant as a whole. The distribution of material is an important instrument in particular for adjusting the gas distribution.
In this respect, DE-C-19623246 describes an apparatus for the common, central introduction of coal and sponge iron into a melter gasifier. Although suitably thorough mixing of the substances is achieved, the central introduction of the coal/sponge iron mixture has not proven advantageous, for both process engineering and economic reasons.
In view of the prior art, it is an object of the present invention to further develop a process using an apparatus so as to achieve a more economical process and a more economical plant engineering design compared to the prior art.
The present invention has proven particularly advantageous when used in a melter gasifier and is documented in greatest detail in this connection. However, the use of the invention is not restricted to this embodiment, but rather the description of the actions in a melter gasifier merely represent an explanation given by way of example.
A melter gasifier, as is known in the prior art, is used to melt an iron ore which has been largely prereduced (DRI), and to produce reduction gas, preferably from coal.
The coal and the DRI are generally introduced into the melter gasifier via the dome of the gasifier; it has proven expedient for the coal to be introduced centrally. Accordingly, the DRI is introduced into the melter gasifier via a plurality of eccentrically located openings at the gasifier dome.
The invention is also characterized by a process for producing a fixed bed in a metallurgical unit, preferably for producing pig iron or primary steel products from iron-containing charge materials, in particular in a melter gasifier, in which a lumpy bulk material, which contains ore-containing and carbon-containing constituents, in particular prereduced iron ore, preferably sponge iron, and, preferably lumpy, coal, is charged onto a surface, and thorough, preferably uniform mixing of the ore-containing constituent with the carbon-containing constituent of the bulk material takes place, in which method the entire ore-containing constituent is charged onto an active circumferential region (peripheral region) of the fixed bed, at which the thorough, preferably uniform mixing of the ore-containing constituent with the carbon-containing constituent of the bulk material takes place.
In this context, the active circumferential region refers to that region of the fixed bed through which gases pass uniformly in sufficient quantities for the production of pig iron and/or reduction gas.
According to one feature of the invention, a coarse-grained fraction, in particular of the carbon-containing constituent, of the bulk material, which has a mean grain size which is greater than the mean grain size of the bulk material which is to be distributed, in particular of the carbon-containing constituent, is charged onto the centre of the surface, and in this way a, preferably steady state, predefined grain size distribution is produced.
According to a further feature of the invention, the bulk material, in particular the carbon-containing constituent of the bulk material, is distributed, via a charging apparatus, in a substantially rotationally symmetrical manner on the surface, less material than the amount which corresponds to the average at the other locations of the surface, between the centre and the outer edge of the active circumferential region of the fixed bed being applied to the centre of the surface by direct distribution.
According to an additional feature of the invention, the coarse-grained fraction, in particular of the carbon-containing constituent, of the bulk material is for the time being applied to the fixed bed at a distance from the centre, in such a manner that, consequently, it is automatically charged onto the centre of the surface by indirect distribution, in particular segregation.
According to a further embodiment of the process according to the invention, the lumpy bulk material is charged via one or more stationary charging apparatuses.
Charging may take place directly or indirectly.
By definition, direct charging means charging in which the bulk material in question, during its introduction, in particular into a reactor or a vessel, is loaded onto a predetermined region of a surface, in particular onto the centre of a surface.
By definition, indirect charging means charging in which, although the bulk material is introduced by direct charging, the resulting distribution over the surface is determined by further effects, in particular by segregation. In this way, it is possible for the bulk material to be distributed and charged in a controlled way onto a specific region of the surface, in particular onto the centre of the surface, even though this region has been omitted, or at least acted on to a lesser extent, by the direct charging, this being achieved for example by segregation i.e. indirectly.
Accordingly, the direct and/or indirect charging establishes a grain size distribution which remains substantially constant as the process continues, i.e. behaves in a steady state, over the surface.
According to one feature of the invention, the expansive surface is a surface through which gases can pass, in particular through which gases actually pass, process gas being guided in a controlled way through this surface. Passage of gas of this type is a significant feature of a corresponding process, for example the passage of gas through the fixed bed of a shaft furnace or melter gasifier.
It is a significant object of the process according to the invention to establish the bed of the gasifier in a suitable way, in order to prevent quantitative, pressure and analytical fluctuations in the gas system above the bed. Since a melter gasifier, as well as generating the pig iron, is also used to produce reduction gas, irregular gas flows significantly impede its operation. These irregularities may even lead to the formation of gas fountains, which lead to a sudden expulsion of dust from the unit. The discontinuous expulsion of dust, as arises, for example, through sudden carbonization, places a load on the downstream units, in particular a reduction shaft furnace.
Especially in the case of processes in which gas is supplied from the side, below the bed, there is insufficient passage of gases through the centre of the reaction bed given a load in accordance with the prior art. The invention provides countermeasures which significantly improve the process.
The formation of the fixed bed in a melter gasifier differs significantly from the charging of, for example, a blast furnace, since a melter gasifier is on the one hand a unit of a different specification, in particular different dimensions, and, on the other hand, the melter gasifier is operated using a different method, in which different loading means from those used, for example, in the blast-furnace method are used.
In a preferred process of this type, the energy carriers used are carbon-containing solids, in particular coal, and O2-containing gas. According to the prior art, in this case the coal is conveyed out of a coal bunker using one or more worm conveyors and is added centrally, the coal therefore dropping in a narrow, concentrated jet through the gas chamber of the melter gasifier onto the bed surface. Furthermore, it is also conceivable for the coal not to be added centrally to the fixed bed, but rather separately via a plurality of part-streams.
Working on the basis of the central introduction of the coal into the gasifier, the coal, on account of the characteristic of the worm conveying, does not drop onto the centre of the bed surface, but rather drops slightly eccentrically, on account of the horizontal velocity of the worm discharge.
On account of the tendency for the charge to accumulate at certain points, and on account of relatively fine particles and the tendency of the coal to agglomerate, the passage of gas through the bed deteriorates at the central charging points. A cone of bulk material is formed and from time to time different volumes of this cone slip down suddenly into the circumferential region through which gases are passing. The coal passes into the hotter surrounding region and in the process is carbonized very rapidly.
Quantitative gas fluctuations with pressure influences and analytical fluctuations are the consequence, resulting in further adverse effects on the downstream gas system.
Furthermore, this slipping of the coal leads to an uneven and asymmetrical distribution of material at the circumference. Continuous heating of the burden is disrupted as a result, so that directly reduced iron (DRI) is heated to different extents at the circumference, and consequently it is impossible to establish a uniform temperature profile. Fluctuations in the quality of pig iron and slag are the result. Local differences in the slag composition at the circumference lead to disruption in the outgoing flow, and the desired slag composition in the hearth can only be established to an insufficient extent by mixing of the charge materials.
The punctiform charging of coal into the central region of the bed surface which is customary when loading the melter gasifier consequently leads to an uncontrolled formation of the bed surface and, depending on the segregation behaviour, to an unfavourable distribution of the various grain sizes of the bulk material.
Preferably, with charging of this nature, the larger grain will move outwards. The gas which flows through the bed from below consequently tends to be forced towards the wall of the gasifier and to be distributed in an uncontrolled manner through the fixed-bed cross section. High local gas velocities which may even lead to fountains being formed disturb the gas reactions in the gasifier dome and increase the discharge of dust. Consequently, there is a large area in the centre of the gasifier through which little gas flows. The volume of the active bed is therefore reduced, and the dead man in the centre or in the hearth is primarily supplied with relatively fine grains, so that the drainage deteriorates further. The dead man is a solid column shaped area of coal in the center area of the melter gasifier which forms due to insufficient oxygen in the center area to gasify the solid column shaped area of coal.